The present invention relates to an optical pickup head apparatus for scanning an object with a focused beam to read out information stored in the object with high density, and more particularly to an optical pickup head apparatus for an optical disk.
In the optical pickup head apparatus above-mentioned, the half width .epsilon. of a focused beam basically places limits on the information reading function. Here, .epsilon. is given by the wavelength .lambda. of a light source and the numerical aperture NA of an objective lens according to the following equation: EQU .epsilon.=.alpha..times.(.lambda./NA) (1)
wherein .alpha. is a constant to be determined by the opening shapes of the apertures of the objective lens and the complex amplitude distribution of a beam at the aperture of the objective lens.
Generally, the information-recorded surface of an optical disk is formed by discrete patterns 50a, 50b, 50c, . . . made in the form of pits as shown in FIG. 21. The beam size of a reading beam 50A is determined by the equation (1). Accordingly, when .epsilon. is not sufficiently small as compared with a track pitch P.sub.t, the reading beam 50A also reads out signals of adjacent tracks. Such crosstalks disadvantageously prevent a signal from being accurately read out in a stable manner.
To suppress such crosstalks, there is proposed an optical pickup head apparatus capable of reading high-density information, as disclosed in Japanese Patent Laid-Open Publication No. 57-58248. In this optical pickup head apparatus, there are used, as reading beams, three beams, i.e., a first beam 50A which is located in the center, and second and third beams 50B and 50C which are respectively located at both sides of the first beam 50A, as shown in FIG. 21. The second and third beams 50B, 50C are separated from the first beam 50A by a track pitch P.sub.t in directions at right angles to the tracks.
The optical pickup head apparatus above-mentioned has an arrangement as shown in FIG. 22. More specifically, there are used, as light sources, a first semiconductor laser 51, a second semiconductor laser 52 and a third semiconductor laser 53. A beam emitted from the first semiconductor laser 51 passes through a first beam splitter 54, a collimate lens 55 and a second beam splitter 56. Then, the beam is focused by an objective lens 57 and reaches the information-recorded surface of an optical disk 58. A beam emitted from the second semiconductor laser 52 and a beam emitted from the third semiconductor laser 53 pass through a third beam splitter 59, the first beam splitter 54, the collimate lens 55 and the second beam splitter 56. Then, these beams are focused by the objective lens 57 and reach the information-recorded surface of the optical disk 58. Three beams emitted from the first, second and third semiconductor lasers 51, 52, 53 form images on the information-recorded surface of the optical disk 58 with a predetermined positional relationship. The reflected lights of these beams return back to the first to third semiconductor lasers 51, 52, 53, respectively, where the amounts of the beams serving as signals are read out by the self-coupling effects of the semiconductor lasers 51, 52, 53.
When it is supposed that the amount of a crosstalk of each of the first to third beams 50A, 50B, 50C with respect to adjacent tracks is set to k, the amounts of signal lights S.sub.A, S.sub.B, S.sub.C respectively obtained from the first to third beams 50A, 50B, 50C are expressed according to the following equations: EQU S.sub.A =k S.sub.-1 +S.sub.0 +k S.sub.+1 ( 2) EQU S.sub.B =k S.sub.-2 +S.sub.-1 +k S.sub.0 ( 3) EQU S.sub.C =k S.sub.0 +S.sub.-1 +k S.sub.+2 ( 4)
wherein S.sub.-2, S.sub.-1, S.sub.0, S.sub.+1, and S.sub.+2 refer to the amounts of signal lights obtained when beams are incident upon the centers of the tracks -2, -1, 0, +1, +2, respectively. In the equations above-mentioned, the amounts of signal lights from second adjacent tracks which are so small, are disregarded.
From the equations above-mentioned (2), (3), (4), the following equation is obtained as an equation of operational processing for suppressing a crosstalk: ##EQU1##
The amount of a crosstalk at the time when reading the amount of a signal light by a single beam, reaches the level of 2 k. However, when an operational processing is executed according to the equation (5), the amount of a crosstalk can be reduced to 2 k.sup.2 /(1-2k.sup.2), i.e., the level of 2 k.sup.2. Accordingly, the track pitch P.sub.t of pits to be recorded, can be narrowed.
To accurately focus, on the information-recorded surface of the optical disk 58, the first to third beams 50A, 50B, 50C which are put close to one another with the track pitch P.sub.t above-mentioned, it is required that beams emitted from the three independent first to third semiconductor lasers 51, 52, 53 are adjusted with high precision and put in a single luminous flux. On the other hand, the beams which are reflected and returned, as partially overlapping one another, from the optical disk 58, cannot be individually guided to independent photodetectors. Accordingly, the signals are read out by the self-coupling effects of the first to third semiconductor lasers 51, 52, 53.
When reading the amounts of signal lights, it is difficult to avoid the influence of noise by return lights generated in the first to third semiconductor lasers 51, 52, 53. This prevents the acquirement of highly precise signals each having a stable S/N ratio.
In this connection, there is proposed a method of reading the information of adjacent tracks with the use of independent photodetectors, as shown in "Multi-Beam Optical Disk Drive for High Data Transfer Rate Systems", Proc. Int. Symp. on Optical Memory, 1991, pp. 268-272, by R. Katayama et al. More specifically, as shown in FIG. 23, there are provided a sufficient distance 1 in the tracking direction between a main beam 60 and each of sub-beams 60A, 60B, and output signals obtained by photoelectrically converting the main beam 60 and the sub-beams 60A, 60B are subjected to an operational processing with the use of an adaptive digital filter, thereby to suppress regenerated signals from adjacent tracks, as well as intersymbol interference in the recording line density direction.
In the method above-mentioned, however, it is required to adjust the spot positions of the main beam 60 and the sub-beams 60A, 60B such that a track is scanned by the main beam 60 and the sub-beams 60A, 60B with the distance 1 (which is not less than several tens .mu.m) between the main beam 60 and each of the sub-beams 60A, 60B accurately maintained at both inner and outer peripheral portions of an optical disk. This requires a complicated optical system and a complicated control mechanism. This is disadvantageous in view of stabilization and miniaturization of an optical pickup head apparatus. Further, there are also required timing control for compensating time delays between the main beam 60 and the sub-beams 60A, 60B, as well as other complicated signal processings.